It's a chapter from here: Rumelhart, D. E., McClelland, J. L., & the PDP research group. (1986). Parallel distributed processing: Explorations in the microstructure of cognition. Volume I. Cambridge, MA: MIT Press.

The whole book is quite good, btw! And it looks cool on a bookshelf.

Another version of that paper [0] is dated Feb. 1986.

[0]: https://apps.dtic.mil/sti/tr/pdf/ADA620727.pdf